This invention relates to copper workpieces having smaller pore size than hitherto obtainable and to a method of making such workpieces.
Porous plugs having pore sizes smaller than 10-15 microns are required as phase separators in vent lines of long hold-time liquid helium containers in a zero-gravity environment. Such containers are needed for experiments employing the cryogenic technology developed around superconductivity and the Josephson effects. In order to maximize liquid helium hold-time (and hence the lifetime of the experiment), it is essential to ensure that none of the liquid helium is lost through the vent lines along with the vapor, owing to liquid flotation in the conditions of low gravity.
Insertion of a porous plug separator into the vent line has been proposed as a solution to this problem. P. M. Selzer et al., "A Superfluid Plug for Space," Advances in Cryogenic Engineering, Vol. 16, p. 277, New York, Plenum Publishing Corp., (1971); E. W. Urban et al., "A Porous Plug for Control of Superfluid Heliumin Space," Proceedings of the Second Annual Research and Technology Review, George C. Marshall Space Flight Center, presented by the Research and Technology Office, Science and Engineering Directorate, NASA, p. 424, October 1974.
In addition, because of special properties of liquid helium below its superfluid transition at T = 2.18 K, a porous plug permits thermal control of the liquid. Such a plug should have a high thermal conductivity, and the channels through the plug should be large enough to maintain a flow rate below the critical velocity of superfluid helium in the channels yet small enough so that the heat conducted by the liquid is much less than the heat conducted by the plug material.
The usual procedures for making high thermal conductivity porous plugs is by "sintering" powdered metal. In this process suitably compacted metal powder is heated to just below the melting point of the metal, and the powder grains fuse together to form a porous solid. The average pore diameter in this porous solid is largely determined by the average diameter of the metal granules in the powder. A shortcoming of this technique is that channel size and the ratio of the average total area of the channels to the average cross-section of metal cannot be controlled independently. The smallest average pore size in plugs available for testing to date is 10 to 15 microns, although smaller pore sizes had been sought. Although ceramic plugs with pores 1 to 2 microns in diameter are available, ceramic materials have very poor thermal conductivity at liquid helium temperatures.
Todd, in U.S. Pat. No. 3,276,919, teaches that metal structures having very fine pores can be obtained from structures having courser pores, e.g., bundles of tungsten wires or compacted particles, by oxidizing the porous metal structure to coat each layer or particle with adherent metal oxide, which occupies a greater volume than the metal, and reducing the oxide to metal without contraction in volume to produce a porous cellular metal structure of greatly increased surface area. The Todd process is therefore a modified sintering process.
Synder et al (U.S. Pat. No. 3,546,029) shows the use of a reducing gas to remove heavy surface oxide or scale from a copper rod, without any effect other than at the surface.
Hess (U.S. Pat. No. Re. 26,960) prevents the formation of scale on steel or copper by heating with a stoichiometric mixture of fuel and combustion supporting gas containing oxygen until the metal reaches scaling temperature, changing to a fuel-rich mixture, and heating to the desired temperature. No interior modification is noted.
Thus, available technology for making metal workpieces of very small pore size is based on modification of substrates already having, to some extent, a porous structure and does not provide the very small pore sizes, below 10-15 microns, required for plug separators in vessels for cryogenic use.